Intermodulation (IM) is the production in an electrical device of frequencies that are the sums or differences of frequencies of different inputs or of their harmonics. Reverse intermodulation (IM) products are unintended signals that are produced by the mixing of harmonics of an unwanted signal received on an output of the electrical device with the fundamental frequency of a primary signal being output on the electrical device. The output signal can include reverse IM products as well as the primary signal. As the frequency spectrum gets more crowded, due to the demands and therefore the deployments of more wireless communication, imaging, and sensor systems, the issues with reverse IM products become more pronounced and damaging to the primary signal or to unintended recipients of the output reverse IM products.
Reverse IM products are due to the reception of nearby external signals as is illustrated in FIG. 1. For example, a transmitter 20 injects some of its energy, having a frequency fb, into a nearby transmitting antenna 22 of a transmitter 24, which transmits a primary signal at frequency fa. The external energy with frequency fb travels in the reverse direction from the antenna 22 of transmitter 24 toward an output port 26 of a power amplifier 28 of transmitter 24. The unwanted energy from transmitter 20 mixes with energy emerging from nonlinear transistors inside power amplifier 28 of transmitter 24 to create a number of IM products.
By definition, the IM products are formed by the mixing of the fundamental frequencies and harmonics of the primary and received external signals, i.e., signals having frequencies fa and fb in this example. The more damaging IM products are those that have frequencies near the fundamental frequencies, because such IM products usually cannot be easily filtered out without the use of extremely high-Q filters, which are very expensive if they even exist. For example, third-order IM products are formed by the mixing products 2fa-fb and 2fb-fa. Similarly, the fifth, seventh, and ninth orders are formed by 3fa-2fb and 3fb-2fa, 4fa-3fb and 4fb-3fa, and 5fa-4fb and 5fb-4fa. Higher order IM products also exist and add to spectral congestion and can interfere with systems being deployed. For the sake of clarity, only the third-order IM products are depicted in FIG. 1, but it must be noted that higher order IM products may also exist and can cause issues.
These IM products get propagated by the antenna 22 associated with transmitter 24 and pollute the transmitter's output spectrum with undesired spectral content, which may interfere with other transmitters and receivers that happen to be within a path of propagation of energy from the antenna 22 of transmitter 24. The propagated IM products can interfere with nearby external electronic systems, and may cause these systems to cease from functioning properly.
FIG. 2 illustrates a frequency spectrum of the energy transmitted by transmitter 24, including the primary signal at frequency fa, one interfering signal at frequency fb, and several IM products. Representative third, fifth, seventh, and ninth order IM products above the primary frequency are identified as IM3a (2fa-fb), IM5a (3fa-2fb), IM7a (4fa-3fb), and IM9a (5fa-4fb). Representative third, fifth, seventh, and ninth order IM products below the primary frequency are identified as IM3b (2fb-fa), IM5b (3fb-2fa), IM7b (4fb-3fa), and IM9b (5fb-4fa). The frequencies of the signal components are shown as a function of relative signal power level for a primary frequency, fa, of 30.5 MHz and an interfering signal, fb, having a fundamental frequency of 30 MHz.
Referring again to FIG. 1, external receiver 30 may be located along the propagation path of the primary signal transmitted by transmitter 24. Receiver 30 then becomes an unintended receiver of the signal transmitted by transmitter 24. Receiver 30 may have a receive channel including, for example, the frequency fc=2fa-fb (IM3a) and thereby may receive this third order IM product from transmitter 24.
The IM products near the fundamental frequencies are difficult to deal with because such IM products usually cannot be easily filtered out without the use of extremely high-Q filters, which are expensive, if they exist. Furthermore, there may be a second transmitter (or more), such as interfering transmitter 32 illustrated in dashed lines in FIG. 1. Transmitter 32 may transmit energy having a fundamental frequency fd that is also received by the antenna 22 of transmitter 24. This second interfering signal may cause IM products to be produced that involve fundamental and harmonic signals of all three signals fa, fb, and fd.
With the deployment of more wireless systems and services in densely populated areas, it is very probable that more than one tone can interfere with the transmitter of interest. The higher the number of interference tones, the more IM products are produced and thus the worse the spectral pollution is.